The present invention relates to an active matrix substrate, and flat panel display device and image sensing device using the same.
A liquid crystal display device, as is generally known, holds liquid crystal molecules in between two substrates on which electrodes are formed, and electric signals applied to between the electrodes on the both substrates vary the transmissivity of light coming from back light, by which information is displayed. Compared to cathode-ray tube display devices, the liquid crystal display devices are thin, light-weighted, and low power consumption, and mounted on such devices as desktop personal information terminal equipment and amusement instruments.
As demands for high fineness and high image quality are increasing, a prevailing liquid crystal display device is the one of an active matrix type having active elements such as thin-film transistors (hereinafter referred to as TFT). In the field of the active matrix-type liquid crystal display device, much development efforts for increasing the aperture ratio of a pixel is made. This is because increase of the aperture ratio improves the transmissivity of incident light from back light, so that the same illuminance can be obtained with decreased power consumption and with the same power consumption, higher illuminance can be obtained.
As such an active matrix-type liquid crystal display device with increased aperture ratio, there has been proposed, for example in Japanese Patent No. 2933879 which belongs to an applicant of the present invention, a liquid crystal display device in which a pixel electrode is extended all over an aperture portion. One of the pixels formed on a substrate in the shape of a matrix is shown in FIGS. 7 and 8. FIG. 7 is a plan view of an active matrix substrate, and FIG. 8 is a cross sectional view of FIG. 7 taken on line 8—8.
The active matrix substrate of the liquid crystal display device is mainly configured as follows. On a light permeable substrate 21, there are provided a TFT 14 serving as a switching element, a gate signal line 12 for controlling the TFT 14, a source signal line 13 connected to the TFT 14 and formed orthogonal to the gate signal line 12, an interlayer insulating film 28 formed on the TFT 14, the gate signal line 12 and the source signal line 13, and a pixel electrode 11 to be connected to the TFT 14 through a contact hole 16 passing through the interlayer insulating film 28.
The active matrix substrate of the liquid crystal display device is fabricated by the following process. First, a gate signal line 12 and a capacity line 17 are formed on a light permeable substrate 21, and a gate insulating film 23 is formed so as at least to cover those lines 12 and 17. After that, at the place for forming a TFT 14, there are created a semiconductor layer 24, a channel protection layer 25 as needed, a source electrode 26a, and a drain electrode 26b. Then, there are formed a source signal line 13 to be connected to the source electrode 26a, and a connection electrode 15 to be connected to the drain electrode 26b, and thereafter an interlayer insulating film 28 is formed over the entire surface of the substrate. Further, in order to establish contact between a pixel electrode 11 formed on the interlayer insulating film 28 and the connection electrode 15, a contact hole 16 is provided in the interlayer insulating film 28. Thus, the liquid crystal display device is formed. It is noted that part of the connection electrode 15 and the source signal line 13 are, as shown in FIG. 8, formed by laminating a transparent conductive line 27a and a metal line 27b. 
The pixel electrode 11 is formed by the following steps. First, the interlayer insulating film 28 and the contact hole 16 are formed, after which a transparent conductive film such as ITO (indium tin oxide) films is formed by a sputter technique and the like. On the transparent conductive film, positive type resist is applied by spin coating method. Next, while alignment with the gate signal line 12 and the source signal line 13 is performed, an exposure mask is set by an exposure device such as steppers, and exposure is made from the upper side. Then, the transparent conductive film is etched in accordance with an exposed pattern to complete the pixel electrode 11.
On the other hand, there is a method disclosed by the applicant of the present invention in Patent Laid-Open Publication HEI 10-20321, in which ITO by the spatter technique is not used in creation of pixel electrodes, but ITO materials enabling the formation of film by coating method are used to thicken the pixel electrode 11 at the position of the contact hole 16 so that rubbing errors and display errors are prevented by planarization. However, although there is a difference that an ITO film to be served as the pixel electrode 11 is formed by the coating method, this method is the same as the above stated fabrication method in the point that a patterning process of the ITO film uses a photo lithography technique and an etching technique.
It will be understood that the above-stated active matrix substrate having pixel electrodes formed on an interlayer insulating film is used not only in flat panel display devices such as liquid crystal display devices but also in flat panel image sensing devices as disclosed for example in “A New Digital Detector for Projection Radiography”, Proc. SPIE, Vol. 2432, pp. 237–249, 1995 by Denny L. Lee, et al.
However, the conventional active matrix substrate described hereinabove has following problems.
i) Creation of the pixel electrode 11 requires a series of steps after an ITO film is fabricated by the above-stated method over the entire surface of an substrate, the steps including application of photo resist, mask exposure and development of the photo resist, etching of the ITO film, and removal of the photo resist, resulting in long fabrication process of the pixel electrode 11.
ii) In the process of patterning an ITO film, when photo resist applied on the ITO film is mask-exposed, dispersion of exposure precision (pattern accuracy of photo resist) within the substrate causes corresponding dispersion of parasitic capacitance generated in a superposed portion of the pixel electrode 11 and the gate signal line 12 or in a superposed portion of the pixel electrode 11 and the source signal line 13. The dispersion of the parasitic capacitance affects the uniformity of display in display devices. Upon exposure of photo resist especially by a stepper exposing device, the parasitic capacitance shows slight variance per shot of the stepper, resulting in facilitated generation of irregular display per shot.
iii) In the process of patterning an ITO film, when positive type photo resist applied on the ITO film is mask-exposed, foreign matters such as dust attached on the substrate or on the mask hinder the photo resist of the attached area from being exposed, as a consequence of which the attached area is left as an unnecessary resist pattern. If the unnecessary resist pattern is present in a clearance portion between adjacent pixel electrodes, the ITO film of the portion is not etched and remained in an etching step thereafter, which may establish electrical connection between the pixel electrodes, causing leakage failures.